Stirling engines for driving a generator are disclosed in JP 64-75865 A and JP 2002-266701 A. The Stirling engine disclosed in JP 64-75865 A drives a refrigerant compressor included in a heat pump, and a generator. When the engine speed of the Stirling engine changes due to changes in air-conditioning load and the atmospheric condition, a field regulator included in the generator regulates the magnetic field of the generator to control the engine speed of the Stirling engine. Consequently, a change in load on the heat pump is compensated by electric load to maintain the load on the Stirling engine constant.
The Stirling engine disclosed in JP 2002-266701 A uses heat of reaction generated by a catalytic converter placed in the exhaust system of an internal combustion engine, namely, an automotive power unit, for exhaust emission control as a heat source. Mechanical energy generated by the Stirling engine is used for driving a generator and automotive auxiliary machines.
Although the Stirling engine mentioned in JP 64-75865 A uses the heat of a combustion gas produced by a combustor included in the Stirling engine for heating the working gas, nothing is taken into consideration about the use of the brake horsepower of the Stirling engine for reducing the fuel consumption of the combustor. The Stirling engine mentioned in JP 2002-266701 A uses the catalytic converter as a heater. The catalytic converter is heated by the heat of the exhaust gas from the internal combustion engine and the heat of reaction. Since the brake horsepower of the Stirling engine drives the automotive auxiliary machines, and the internal combustion engine does not, the internal combustion engine operates at a low fuel consumption rate and fuel consumption rate for running can be improved. However, in a state where the temperature of the exhaust gas is low, the catalytic converter is not heated at temperatures for its active state, and the brake horsepower of the Stirling engine is low, the auxiliary machines, in some cases, cannot be driven for its normal operation and cannot achieve its functions sufficiently.
Control of the engine speed of the Stirling engine mentioned in JP 64-75865 A through the regulation of load on the generator keeps the load on the Stirling engine driving the refrigerant compressor constant such that the Stirling engine operates at a maximum thermal efficiency. However, an engine speed at which the thermal efficiency of the Stirling engine is a maximum does not coincide with an engine speed at which the brake horsepower of the Stirling engine is a maximum. Usually, the engine speed at which the thermal efficiency is a maximum is lower than that at which the brake horsepower is a maximum. Consequently, the power generating ability of the generator cannot be held at a high level and the generator cannot generate sufficient power.
The present invention has been made in view of such circumstances and it is a principal object of the present invention to reduce the fuel consumption of an internal combustion engine serving as a prime mover that discharges waste heat as a high-temperature heat source for a Stirling engine, and to ensure that auxiliary machines are driven so that its functions can be fully exercised even if the brake horsepower of the Stirling engine is low. Another object of the present invention is to recover the thermal energy of waste heat as much as possible, to further reduce the fuel consumption of a combustion engine, and to ensure that auxiliary machines can be satisfactorily driven even if power supplied from a generator and a battery is not sufficient for properly driving the auxiliary machines.